Replace create_xmodule with create_xblock

@cpennington @nasthagiri please review

* The primary task of this is to do all of the import/export; so, the title's wrong
* reconciles the signature of create_xblock
* uses make_course_key from cale's branch
* uses nimisha's asset fixes
* lots of import/export fixes
what do you mean here "when split_draft supports getting specific versions"?  How does split_draft prevent this, if split supports it?
Also, should you add a `TODO` here since commented out code is submitted?